The 802.11p standard has been proposed for use in the 5.9 GHz spectrum for vehicular safety and commercial use. The FCC has allocated 7 channels of 10 MHz each for this purpose. It is envisioned that vehicles will periodically broadcast safety messages to indicate their position and velocity on the road.
The current 802.11p based DSRC wireless access in vehicular environments (WAVE) systems have a basic safety message format where vehicles periodically announce their position, velocity, current operating status, and/or other attributes to other cars allowing the neighboring traffic to track the vehicles' positions and avoid collisions, improve traffic flow, etc. The standard does not preclude pedestrians from utilizing this spectrum and periodically transmitting basic safety messages which can indicate the pedestrians' presence to vehicles around them. However, the spectrum allocated for safety messages is different than that normally used by cell phones for voice communications.
Typically, in vehicular systems, the basic safety messages are transmitted and received periodically in a reserved channel, e.g., a safety channel or control channel, and the transmission periodicity can be as high as once every 50 milliseconds. For vehicular systems, this frequency may not be an excessive burden on the battery or the channel resources.
However, transmitting safety messages in the spectrum allocated for safety messages too often by a pedestrian's phone, assuming the phone includes a transmitter capable of using the spectrum allocated for safety messages, can be a drain on the phone's battery. In addition, the 802.11p spectrum for safety messages can become congested with little or no benefit if a large number of pedestrians' phones send out safety messages which are of little practical use, e.g., because the pedestrian is not close to a road or traffic.
The various sensors (e.g., inertial guidance sensors) and GPS measurement modules of vehicular systems may also be constantly active to provide low-latency position and inertial information. Since the devices are powered by a vehicle, power consumption is not a critical issue. However, in a cellular phone, such operations may rapidly deplete the battery of the cellular phone. For example, a wireless local area network (WLAN) (e.g., Wi-Fi) module, a GPS module, and (to a lesser extent) various inertial sensors of the cellular phone may consume a significant amount of energy. As a result, the battery may be depleted in a few hours and, consequently, a user may be required to completely disable a feature or may run out of battery power too soon.
To enable receipt of safety messages communicated in the spectrum allocated for safety messages may require an 802.11p radio to be switched on for significant durations of time and can be a burden in terms of consumption of battery power. Thus, keeping the receiver on, with regard to safety message operations, in an environment where a pedestrian user is not interacting with road traffic can lead to an unproductive use of battery resources.
Furthermore, a large number of pedestrian users who are not actively using a road may quickly congest the use of a safety channel. For example, the cell phones of driver and passenger of a vehicle may both transmit safety messages which are likely to be redundant when the vehicle in which the driver and passenger are located transmits safety messages.
In view of the above discussion, it should be appreciated that there is a need for methods and apparatus for controlling whether and/or when a device will transmit safety messages. It should be appreciated that it would be desirable if at least some methods and/or apparatus reduced and/or avoided the transmission of safety messages that are not likely to be useful and/or which provide redundant or similar information.